This invention is directed in general to steam turbine controls for power plants which include a steam turbine bypass capability and sliding pressure operation; and, in particular, this invention provides for the improved operation and efficiency of a combined cycle power plant.
In 1975, U.S. Pat. No. 3,879,616 issued to Baker, Dimitroff, Mizen and Gray, assigned to the assignee of the present invention, described the operation of a combined cycle power plant. In particular, the patent describes the operation of the steam turbine under load and pressure control whereby at a particular constant pressure, the turbine main control valve position is adjusted to accept increased steam flow while maintaining the constant steam pressure. As the valve attains 95% open, a higher pressure setpoint is introduced causing the valves to reposition to a new, less open position, about 70% open, whereupon the pressure is held constant while the valves are reopened to a accommodate new flow. This cyclic action is repeated several times until full load, flow and pressure are attained at valves wide open. Conversely, the steam turbine is unloaded by holding pressure constant as flow is diminished until a valve trigger point is reached, at which point a lower pressure level is attained by reopening the valve.
The present invention improves upon the foregoing patented invention by allowing the pressure to rise as flow becomes available and, hence, by putting the turbine in a boiler following mode. The pressure rise is not stepped or jogged as in the prior art patent but rather accomplished by raising the main control valve to a "normal set" or almost wide open condition at some predetermined flow-pressure, and then as flow continues to increase, adjusting the header pressure setpoint in accordance with the actual header pressure. This is accomplished through a unique feed-forward loop, through a split range signal controller and low valve gate, valve position sensing and feedback to the pressure setpoint circuit. The advantages obtained are more efficient operation, because of less valve throttling, more efficient design through the elimination of parallel independent control loops and more reliable operation because of less valve cyclic operation. Further, the invention provides for a smooth transition between bypass pressure control to main control valve pressure control.
Accordingly, one object of the present invention is to provide an improved boiler following, sliding pressure turbine control system.
Another object of the invention is to provide an improved turbine control system which will obviate ramping valves open and closed to obtaining varying pressure levels.
Still another object of the present invention is to provide a turbine control system which provides interdependent control loops.
Other objects, advantages and features of the present invention will become apparent from the following description of a preferred embodiment thereof when read in connection with the accompanying drawings.